Big Boss adaptor is used to connect a smaller diameter waste, overflow, or condensate pipe into a larger diameter soil pipe or downpipe. This is particularly useful in retrofit applications where access might be limited, preventing the use of a traditional strap-on boss. 
Common applications include:
Connecting a boiler's condensate pipe to a rainwater downpipe or soil stack.
Adding an overflow pipe from a toilet cistern or system tank to a soil pipe.
Connecting standard waste pipes (32mm, 40mm, 50mm) into a 110mm soil pipe. 
Installation
The installation process typically involves cutting a specific size hole into the existing pipe (using a holesaw), applying a solvent weld adhesive around the hole and the adaptor's inner face, and then pushing the adaptor in to create a secure, water-tight connection.